1. Field of the Invention
The present invention relates to a highly reliable semiconductor laser and a method for producing the same which requires relatively small power consumption.
2. Description of the Related Art
Various techniques have been explored for providing a highly reliable semiconductor laser which requires relatively small power consumption. For example, Japanese Laid-Open Patent Publication No. 5-160503 discloses semiconductor laser device having a ridge guide structure of a so-called actual refractive index guide type.
Referring to FIG. 8, the above-mentioned conventional semiconductor laser is described. This semiconductor laser includes an n-GaAs buffer layer 202 (thickness: about 0.5 .mu.m), an n-Al.sub.0.5 Ga.sub.0.5 As lower cladding layer 203 (thickness: about 1 .mu.m), an n-Al.sub.0.15 Ga.sub.0.85 As active layer 204 (thickness: about 0.07 .mu.m), a p-Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 221 (thickness: about 1 .mu.m in a ridge portion and about 0.15 .mu.m in regions other than the ridge), a p-GaAs cap layer 208 (thickness: about 0.2 .mu.m), an n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 (thickness: about 0.5 .mu.m), an n-GaAs protection layer 220 (thickness: about 0.5 .mu.m), and a p-GaAs contact layer 212 (thickness: about 3 .mu.m) formed on an n-GaAs substrate 201. A ridge 213 is formed in the upper cladding layer 221.
Hereinafter, a method for producing the above-mentioned conventional semiconductor laser is described with reference to FIGS. 9A to 9D.
First, in the first stage of semiconductor layer crystal growth shown in FIG. 9A, the n-GaAs buffer layer 202, the n-Al.sub.0.5 Ga.sub.0.5 As lower cladding layer 203, the Al.sub.0.15 Ga.sub.0.85 As active layer 204, the p-Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 221, and the p-GaAs cap layer 208 are sequentially formed on the n-GaAs substrate 201 (Step A).
Next, as shown in FIG. 9B, a dielectric film 216 (e.g., nitride or silicon oxide film) is formed in a striped shape on the p-GaAs cap layer 208. By using the dielectric film 216 as a mask, the p-GaAs cap layer 208 and the p-Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 221 are partially etched away, thereby leaving a ridge 213 (Step B).
Next, in the second stage of semiconductor layer crystal growth shown in FIG. 9C, by using the dielectric film 216 again as a mask, the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 and the n-GaAs protection layer 220 are-selectively grown so as to be present only on the side faces of the p-GaAs cap layer 208 and on the p-Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 221 by MOCVD (metal organic chemical vapor deposition) (Step C).
Next, as shown in FIG. 9D, the dielectric film 216 is removed. In a third stage of semiconductor layer crystal growth, the p-GaAs contact layer 212 is formed so as to cover all of the portions of the p-GaAs cap layer 208, the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209, and the n-GaAs protection layer 220 that are exposed on the surface (Step D).
Finally, an electrode is formed on each of the n-GaAs substrate 201 and the p-GaAs contact layer 212 to complete the semiconductor laser. The above-described conventional semiconductor laser achieves laser oscillation by confining light and an electric current in a region below the Al.sub.0.15 Ga.sub.0.85 As active layer 204.
As described above, the above conventional semiconductor laser has a ridge guide structure where a portion of the p-Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 221 defines the ridge 213. At Step B, the p-Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 221 is inevitably exposed to the atmosphere after the formation of the ridge 213. In general, a portion of an AlGaAs layer exposed to the atmosphere forms a deep energy level because of its Al content, which is an element relatively susceptible to oxidation. This causes the AlGaAs layer to absorb some light, thereby degrading the reliability of the semiconductor laser. In contrast, in the above-mentioned conventional semiconductor laser, the portion which was actually exposed to the atmosphere is located at a certain distance from the active region thereof, so that the unfavorable light absorption occurring in such exposed portions is substantially reduced. Thus, the above-mentioned conventional semiconductor laser attains a high reliability.
Moreover, the conventional semiconductor laser confines light along the horizontal direction only in terms of the actual refractive index, i.e., by providing the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 (having a smaller refractive index than that of the p-Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 221) on the outside of the ridge 213 of the p-Al.sub.05 Ga.sub.0.5 As upper cladding layer 221. Since this structure (often referred to as the "actual refractive index guide type structure") attains the horizontal confinement of light without utilizing light absorption, it can reduce the propagation loss during laser oscillation, which in turn reduces the power consumption of the laser.
In the above-mentioned method for producing a conventional semiconductor laser device, the dielectric film 216 is layered above the ridge 213 during the step of forming the ridge 213. During the subsequent MOCVD growth, the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 and the n-GaAs protection layer 220 are selectively formed only in regions other than in the dielectric film 216. As a result, the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 and the n-GaAs protection layer 220 are prevented from being formed on the ridge 213 during the second crystal growth.
Moreover, the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 and the n-GaAs protection layer 220 of the conventional device are formed during the second crystal growth. Although the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 would suffice to confine light and an electric current, the n-GaAs protection layer 220 ensures that GaAs (which is relatively immune to oxidation) extends over a large region of the surface of the device before the third crystal growth. Thus, the crystallinity of the p-GaAs contact layer 212 formed through the third crystal growth improves as compared with the case where the n-GaAs protection layer 220 is not provided.
HF (hydroxy fluoride) is generally used for etching away the dielectric film 216. In the conventional semiconductor laser device, the n-GaAs protection layer 220 provided on the n-Al.sub.0.65 Ga.sub.0.35 As blocking layer 209 makes it possible to use HF for etching away the dielectric film 216. Specifically, HF is capable of etching an Al.sub.x Ga.sub.1-x As layer where x is equal to or larger than 0.4 at a fast etching rate, but the n-GaAs protection layer 220 prevents the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 from being etched during the process of etching the dielectric film 216.
However, the inventors of the present invention attempted producing the above-described conventional semiconductor laser device and tested its operation, which revealed a number of problems which are described below.
Firstly, the aforementioned conventional semiconductor laser device requires a higher driving voltage than those of lasers of other structures existing prior to this device. A study by the present inventors revealed that the crystallinity of the p-GaAs contact layer 212 in regions where it comes in contact with the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 deteriorates so that such regions become highly resistive.
More specifically, in the aforementioned conventional semiconductor laser, the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 and the n-GaAs protection layer 220 are formed during the second crystal growth, thereby ensuring that a large region of the surface of the device before the third crystal growth is GaAs (which is relatively immune to oxidation) so as to improve the crystallinity of the p-GaAsa contact layer 212 formed through the third crystal growth. However, a certain width of n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 is present on each side of the p-GaAs cap layer 208 (defining part of the ridge) on the surface of the laser device prior to the third crystal growth. In the aforementioned semiconductor laser device produced by the present inventors, the width of the above-mentioned portion (appearing on each side of the p-GaAs cap layer 208 on the surface) of the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 was about 0.47 .mu.m, which is only slightly smaller than the thickness (about 0.5 .mu.m) of the portions of the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 present above the p-Al.sub.0.5 Ga.sub.0.5 As upper cladding layer 221 in regions other than the ridge. Owing to the oxidation of the Al content in the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209, the crystallinity of the region of the p-GaAs contact layer 212 (formed through the third crystal growth) on the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 may deteriorate, thereby increasing the resistivity of the region. Since an electric current concentrates in the portion of the p-GaAs contact layer 212 adjacent to the ridge, any increase in resistivity has a large undesired influence in this region, thereby substantially increasing the driving voltage of the semiconductor laser.
Moreover, the n-GaAs protection layer 220 of the aforementioned conventional semiconductor laser, which is provided on the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209, is intended to prevent the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 from being etched during the process of etching the dielectric film 212 with HF. However, the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 is still susceptible to etching by HF in the regions appearing on both sides of the p-GaAs cap layer 208 at the ridge. If the n-Al.sub.0.65 Ga.sub.0.35 As current blocking layer 209 is overly etched, the semiconductor laser may not oscillate.
Secondly, the aforementioned conventional semiconductor laser device has a short life under continuous operation. This problem may be caused by dislocations occurring within the device, which are presumably caused by the use of a nitride film in the production process. Specifically, the production of the conventional semiconductor laser involves an MOCVD selective growth, which utilizes the dielectric film 216 formed on the p-GaAs cap layer 208. The MOCVD selective growth is generally conducted at a temperature from about 650.degree. C. to about 800.degree. C. If a dielectric film formed on a semiconductor layer is exposed to such a high temperature, the semiconductor layer experiences a large amount of stress owing to the difference in the thermal expansion coefficient between the semiconductor layer and the dielectric film, thereby allowing a large number of dislocations to be generated. In general, the reliability of the semiconductor laser substantially decreases as the number of dislocations generated in the laser emission region increases. It is contemplated that the aforementioned conventional laser device includes a number of locations in the laser emission region thereof, thereby resulting in the short operation life thereof.
Thirdly, the MOCVD selective growth of the AlGaAs layer utilizing a dielectric film requires a corrosive gas, e.g., HCl, to be simultaneously used, thereby making it difficult to handle and/or maintain the MOCVD device.